1. Field of the Invention
This invention relates to a color developing ink, and more particularly to a color developing ink which provides a colored image upon reaction with a substantially colorless organic compound.
2. Description of the Prior Art
Colored images have been produced for many years by catalytic reaction between a colorless organic compound as an electron donor (hereunder referred to as a color former) and a solid acid as an electron acceptor (hereunder referred to as a color developer). This technique is effectively incorporated in a pressure sensitive copying paper (disclosed, for example, in U.S. Pat. Nos. 2,505,470; 2,505,489; 2,550,471; 2,548,366; 2,712,507; 2,703,456; 2,730,457; 3,418,250; 3,672,935) and a heat sensitive recording paper (disclosed, for example in Japanese Patent Publication Nos. 4160/68, 7600/68 and 14039/70; U.S. Pat. Nos. 2,939,009; 3,322,557 and 3,539,375). A printing technique is also known for producing a colored image by supplying a color former-containing ink to a sheet coated with a color developer (disclosed in West German Patent Application (OLS) No. 1,939,962).
In a conventional color developer sheet, a color developer is coated over the entire surface of the paper sheet and it is necessary to print a desensitizing ink on the areas of the sheet where color development is not desired and this is not economical. Therefore, a color developing ink is used to print only the desired area of non developer coated sheets. But since the ink uses a low boiling organic solvent, it can only be applied to flexographic or gravure printing and cannot be used in conjunction with general relief or offset printing.
Recently, color developing inks for use in letter press printing have been proposed in Japanese Patent Application (OPI) Nos. 68307/76, 80410/76 and 94308/76, but they are more or less defective in that they do not sufficiently exhibit their color developing ability on the surface to be printed and the light fastness of the colored images produced is weak and impractical, or that they cause swelling of the rubber rollers of a printing machine. Furthermore, objects printed with a conventional developing ink cannot be stacked on a paper coated with color former containing microcapsules for an extended storage without damaging the walls of the microcapsules, impairing their function and, as a result, producing colored stain on the surface coated with the developing ink (the stain will hereunder be referred to as stack-time dependent stain).